T he present study tested two assumptions concerning the auditory processing of microtiming in musical grooves (i.e., repeating, movement-inducing rhythmic patterns): 1) Microtiming challenges the listener's internal framework of timing regularities, or meter, and demands cognitive effort. 2) Microtiming promotes a “groove” experience—a pleasant sense of wanting to move along with the music. Using professional jazz musicians and nonmusicians as participants, we hypothesized that microtiming asynchronies between bass and drums (varying from −80 to 80 ms) were related to a) an increase in “mental effort” (as indexed by pupillometry), and b) a decrease in the quality of sensorimotor synchronization (as indexed by reduced finger tapping stability). We found bass/drums-microtiming asynchronies to be positively related to pupil dilation and negatively related to tapping stability. In contrast, we found that steady timekeeping (presence of eighth note hi-hat in the grooves) decreased pupil size and increased tapping performance, though there were no conclusive differences in pupil response between musicians and nonmusicians. However, jazz musicians consistently tapped with higher stability than nonmusicians, reflecting an effect of rhythmic expertise. Except for the condition most closely resembling real music, participants preferred the on-the-grid grooves to displacements in microtiming and bass-succeeding-drums-conditions were preferred over the reverse.